joshcarter/sudoku

Simple Sudoku solver written in Ruby

Simple Sudoku Solver in Ruby

by Josh Carter
http://github.com/joshcarter/sudoku/tree/master

DESCRIPTION:¶ ↑

Simple Sudoku solver built in Ruby. Run the test.rb for a demonstration, or something like this in irb:

grid = Grid::load("grids/medium_grid.txt")
grid.solve
puts grid

Some details on implementation: Each solved cell contains a single value, each unsolved cell contains an array of possible values. (Coming into initialize() those unsolved cells contain nil.) Taking a cue from Peter Norvig [1], setting any cell causes all peer cells to update their list of possible values. If any of these cells reduce to a single value, that’ll cause its peers to recalculate, and so on. Simple grids can be entirely solved in this manner.

Another important note is that the grid contains a map of every cell’s index to a flat array of all its peers (same row, same column, same zone). This map is computed in initialize() so that the constraint operation (above) can operate very quickly. The peer map is large, so it is not copied in dup(), nor does it need to be, because it’s the same for all grids of the same dimensions.

Early versions of this class were much more dynamic, and would compute peers and possible values on the fly. Doing it that was was elegant (in a way) but also extremely slow – it took an hour to solve the grid in grids/super_hard_grid.txt. Most of the time was spent iterating over the grid, which is what lead me to create the peer map. This version solves the same grid in about 1.2 seconds, roughly a 3000x improvement.

[1]: norvig.com/sudoku.html

LICENSE:¶ ↑

MIT License Copyright © 2008

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the ‘Software’), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sub-license, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED ‘AS IS’, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.